Helminthoses arising and often frequent among farm animals cause significant economic damages. According to estimations the gross profit in the cattle and sheep population is by 20-40 U.S. $ less per capita than would be expected on the basis of its potential. Among the economical losses the damage caused by parasites have an increasingly greater role in addition to the animal keeping, feeding and other veterinary problems. The endoparasites take the nutritive material necessary for their survival and multiplication from the host. The production of farm animals and sometimes even their life is exposed to danger.
The scope of the active ingredients used by the protection against nematodes is wide. These are for example phenotiazine, benzimidazoles, imidazoles, pyrimidines, pyridines, ivermectine, different organic phosphatesters, piperazine, isothiocyanate, certain substituted phenoles and salicylanilides.
The protection against internal parasites is extremely difficult because of the condition that in addition to nematodes, trematodes and cestodes livestock can be infected also by articulata. The articulata belonging to the species of Dipteras are e.g. the horse bot flies (gastrophilidae) causing the bottenness of horses, the gad-flies causing the gad-fly larva disease of cattle skin (hypodermatidae) and the gad-flies (Oestridae) causing the gad-fly larva disease of snout of the sheep.
The current anthelmintics--except ivermectine--are, however, ineffective against Dipteras. Therefore per os administered phosphatetester type insecticides are used against these flies in themselves or combined with an anthelmintic.
Such composition is e.g. Rintal Plus sold by the firm Bayer, which is a mixture of a probendazole--namely phebantel--and trichlorfon in a ratio of 1:5 with an active ingredient content of 42.7 weight %. An analogous composition is Combotel. Another composition containing mebendazole and trichlorfon in a ratio of 8.8:40 is Telmin B. The composition Equizole-B of Merck contains also thiabendazole and trichlorfon in a ratio of 44:40. Mixtures of oxybendazole:trichlorfon are used in similar manner in ratios of 1:3.5 (Vet. Med., 1985, Apr. p. 68) and 1:3 (Vet. Rec., 1986, Sept., p. 294). Another phosphatetester, dichlorvos (Astrobot) is also widely used. Such compositions are Equigard and Equigel (Squibb).
The organic phosphatester insecticides being effective for colinergic synapses are, however, not selective and their oral administration is therefore very hazardous. Their therapeutic index is so low that even double the overdosage thereof has toxic effect. As is known horses are sensitive to treatments with trichlorfon (Vet. Med., 1985, Apr., p. 68) which are accompanied by diarrhoea, frequent defecation and urination. The treatment of animals of poor health and bad condition is therefore contraindicated.
Among insecticides pyrethroids have developed fastest during the last decades. Already more than 20 active ingredients are today at the disposal of agricultural, veterinary and sanitarian workers. But compositions containing a pyrethroid as active ingredient usable against the endoparasite articulata of farm animals are unknown. The possible reason for this is that pyrethroids, as insecticides effective against adult flies and having knock down activity are known and no practical importance has been attached to their activity against larvas of flies and worms.
One of the most important characteristics of the pharmaceutical compositions is the quotient of toxic and therapeutic dosage, the so-called therapeutic index characterizing the security of use. The higher is this value, the smaller is the risk in use, the selective is the active ingredient.